JPH0442478B2 - - Google Patents

Description

[Detailed description of the invention]

A. INDUSTRIAL APPLICATION The present invention relates to the production of thick anodized coatings on aluminum or aluminum alloys, in particular aluminum substrates to obtain substrates suitable for electronic circuit packaging. This invention relates to an electrolytic bath for anodic oxidation. B. PRIOR ART Numerous methods are known in the prior art for anodizing aluminum and aluminum alloys. US Pat. No. 3,985,629 discloses colored anodization of aluminum and aluminum alloys. Anodization is carried out in an electrolytic bath containing sulfuric acid. U.S. Pat. No. 3,849,264 discloses conventional aluminum anodization using an aqueous electrolyte solution preferably containing about 11 to about 17 weight percent sulfuric acid. U.S. Pat. No. 4,229,266 relates to a method of roughening aluminum and aluminum alloys mechanically, chemically, or electrochemically, and then anodizing the aluminum and aluminum alloys in an aqueous electrolyte solution containing sulfuric acid and phosphoric acid. . The concentration of sulfuric acid is about 25 to about 150 g per 1, and the concentration of phosphoric acid is about 10 to about 150 g per 1.
It is 50g. This reference refers to pre-treatments such as grinding, polishing, brushing or sand blasting,
Also discussed are degreasing, pickling, or chemical surface treatments to create a rough surface, or electrochemical surface treatments by the action of an electric current in an acid. The teachings of this document are particularly aimed at the production of materials for use as support substrates for printed circuit boards. US Pat. No. 4,406,757 is directed to a method for electrolytically processing lithographic grade aluminum in a sulfuric acid electrolyte at a temperature below about 50°C. U.S. Pat. No. 4,439,287 relates to a method of anodizing aluminum in a bath at a temperature of about 0 to about 15 degrees Celsius to form a hard, wear-resistant anodized layer with a thickness greater than 4 μm. . U.S. Pat. No. 4,478,689 discloses that aluminum and A method and apparatus for electrolytically treating aluminum alloys is disclosed. US Pat. No. 4,481,083 relates to a method of anodizing aluminum for electrolytic capacitors. US Pat. No. 4,507,179 relates to an aluminum substrate suitable for manufacturing high-density magnetic recording media by anodizing the surface of the aluminum substrate in an aqueous chromic acid solution at an electrolytic voltage of more than 60V. IBM Technical Disclosure Bulletin,
Homola et al., Vol. 23, No. 8, January 1981, pp. 3854-3856, discloses the use of colloidal silica to seal anodized aluminum surfaces. has been done. None of these prior art examples concern a method of producing aluminum, which is particularly used as a substrate for the manufacture of printed circuits, by electrolytic anodization. The above patents and available literature all teach methods in which the anodized coating has a predominantly crystalline hybrid structure. In particular, coatings with a thickness exceeding 25 μm,
It has chemical and mechanical defects and is unsuitable for circuit packaging. Finally, although the above documents are considered relevant to the present invention, there are numerous patents and other prior art related to the anodization of aluminum. However, none of these results in the anodization method of the present invention involving the novel anodization bath (or electrolytic bath for anodization). C. SUMMARY OF THE DISCLOSURE The present invention is directed to a method of forming a thick anodized coating on an aluminum substrate, thereby creating a substrate suitable for use in electronic circuit packaging. The method includes cleaning, rinsing, polishing, anodizing, and rinsing steps. The main novelty lies in the composition of the anodizing solution.
This novel anodizing bath provides an amorphous structure without voids and cracks (mechanical defects), while at the same time eliminating ionic carriers and preventing phase transitions (chemical defects). The composition of this bath is the result of metallurgical and chemical considerations. Problems associated with defects in the form of ionic carriers, voids and stoichiometry are avoided. This polishing bath results in a microetched surface that increases the number of nucleation sites important for a uniform anodized coating. Urea and colloidal silica [LUDOX (EI)
EIdupnot (registered trademark of EIdupnot deNemours & Co.) is a preferred material] which disrupts the growth mechanism and reacts with the non-stoichiometric aluminum oxide.
Prior art methods result in the formation of a hybrid amorphous crystalline structure with voids and cracks. Due to the complexing agent citric acid present in the anodizing bath,
The problems associated with unreacted heavy metals remaining inside the anodized coating source for the ionic carrier are overcome. D Example The anodizing bath solution includes deionized water, sulfuric acid, a complexing agent, a leveling agent, and a stabilizing agent. The composition of the electrolytic anodizing bath that gives the best benefits is 974 ml of deionized water, 20 ml of sulfuric acid, and citric acid as the complexing agent.
5.5g, 3.0g urea as a leveling agent, 6.0ml ammonia-treated colloidal silica as a stabilizer.
including. The time of immersion in the bath depends on the required thickness T.
It changes depending on. This varies based on the average T=15/sec times the total number of seconds. The bath temperature ranges from about 5° C. to about 15° C., and the initial current density ranges from about 320 to about 540 A/m ² . The aluminum workpiece to be anodized is anodized in an anodizing electrochemical cell placed in an electrolyte solution containing the anodizing bath of the present invention. The initial current density and cell voltage were determined 10 to 15 times after introducing the current by setting the voltage dial to the maximum value and gradually increasing the current until reaching the minimum value of 380A/ ^m2 .
Set up within seconds. Such current density corresponds to about 30 to about 35V. During anodization, the current decreases and reaches zero after about 45 to 150 minutes, depending on the size of the workpiece. The voltage increases until it reaches an upper limit determined by the equipment used. In a preferred embodiment of this method, the aluminum workpiece is cleaned using an organic solvent such as a chlorine compound or a weakly basic solution with a pH of 8 to 10 at a temperature of about 50 to about 65°C. After this cleaning step, the workpiece is rinsed with deionized water. The cleaned workpiece is then polished, preferably by the chemical polishing method described in the related patent application. The polishing step is followed by a rinse with running deionized water for less than 1 minute. Anodization is then carried out in a bath solution containing deionized water, sulfuric acid, complexing agents, leveling agents, and stabilizers. After anodizing, the workpiece is heated with deionized water flowing through it.
Rinse for 3 minutes and force dry in air. A particular advantage of the teachings of the present invention lies in the combination of polishing and anodizing recipes. In a preferred embodiment, polishing produces a microetched surface that increases nucleation sites, an important factor in obtaining a uniform anodized coating. The chemical composition of the anodizing bath results in an anodized coating with an amorphous structure without voids or cracks, virtually no ionic carriers, and phase transitions are prevented. The result is an amorphous structure without the voids and cracks that can otherwise spread across the surface of the aluminum. In the well-known anodizing method, a mixed structure such as amorphous and crystalline is formed in aluminum.
This creates voids and cracks. The main difference between the method of the present invention and the prior art method is the use of leveling agents and stabilizers in the anodizing bath. The leveling agent, which in the preferred embodiment is urea, serves to disrupt the growth mechanism. The stabilizer is an ammoniated colloidal silica, preferably Ludox, which reacts with the non-stoichiometric aluminum oxide. The use of ammoniated colloidal silica results in 1) reactions with non-stoichiometric aluminum oxide;
2) prevent phase transition, i.e. from aluminum oxide to aluminum hydroxide or vice versa;
The dual benefits of forming an alumina-silica polymer network eliminate voids and maintain the safety of the coating during heating and cooling. The complexing agent, preferably citric acid, reacts with alloy-forming elements such as copper, iron, and nickel to remove the ionic carrier. Without the use of complexing agents, these heavy metals remain unreacted in the anodized coating. The presence of these metals will cause significant power leakage during the later withstand voltage test. It is further noteworthy that a small amount of sulfuric acid is used in the anodizing bath, preferably 2 to 3% by weight, whereas the prior art
Requires 20% sulfuric acid. Such a dilute sulfuric acid solution of the present invention prevents the dissociation rate of aluminum from increasing, thereby creating a favorable balance of cations of the aluminum workpiece and anions of the electrolyte. This balance creates a dynamic equilibrium and helps the anodized coating to be electrically neutral. The invention can be better understood by the following examples. Most experiments were performed using a commercially available aluminum alloy 6061 containing the following metals as an alloy.
This was done using Si0.6%, Fe0.5%, Mg1.2%,
Cu0.3%, Zn0.2%, Ti0.2%. A suitable anodizing device consists of a DC power source, a tank with a copper anode, two lead cathodes, one on each side, and a stainless steel cooling system connected to a refrigeration system to maintain the anodizing bath at a predetermined constant temperature. It includes a coil. Air was introduced through a plastic perforated tube in the bottom of the anodization tank to maintain a uniform agitation level during the anodization cycle. The anodization console has an instrument panel that monitors and controls current, voltage, time, and air agitation. The anodization temperature was kept constant by a detection device connecting the electrolyte to a cooling device. The anodizing tank was filled with electrolyte, measured in an amount appropriate for the size of the tank. The cooling system was then started to bring the temperature to 10±5°C. Thereafter, the temperature of the anodizing bath was controlled by a detection device that automatically activated the cooling device when the temperature of the anodizing bath exceeded 15°C. During anodization, heat is generated as the oxide layer thickens. An aluminum sample is placed on a copper anode,
A positive bias was applied. Next, I turned the voltage dial and set it to the maximum power output of 75V. The current increases gradually at first and then rapidly until a stable upper voltage limit is reached. In all experiments, the current is
30 to 35A, voltage kept at 35 to 40V. The anodic oxidation time was controlled by a timer, and the apparatus was automatically stopped when a predetermined time elapsed. Typically, current and voltage were held constant during the initial period of anodization (15 to 20 minutes). After that, the current decreased and the voltage increased continuously to the maximum power supply output of 75V. Four samples were intentionally kept in the anodization bath until the current approached zero. Typical anodization times, current densities, and coating thicknesses are shown in Table 1 for 40 samples.

【table】

[Table] Example 1 A total of 50 samples of 12.7 cm x 20.3 cm of aluminum alloy 6061-T6, 974 ml of deionized water, 20 ml of sulfuric acid
ml, citric acid 5.5g, urea 3.0g, Ridox AS-
Anodic oxidation was carried out in a solution containing 6.0 ml of 40. This is the basic recipe for preparing Solution 1. The temperature of the bath is about 5 to about 10°C, and the current density is
It was 0.032A/cm ² to 0.054A/cm ² . The workpiece is
positive in an electrochemical cell. After anodization, the structure and elemental composition of the samples were examined by scanning electron microscopy, X-ray diffraction and atomic absorption. Samples anodized in the bath of the invention have an amorphous structure and a balanced stoichiometry. The elemental composition of the coating was aluminum, oxygen, sulfur, magnesium and silicon. Voltage testing on 30 samples showed that the coating behaved as a true insulator, with a dielectric strength of 700 to 800 volts per mil. Example 2 Post-anodization tests similar to those in Example 1 were carried out on samples anodized using conventional techniques. The results show that in all cases, samples anodized using conventional techniques exhibit a mixed amorphous and crystalline structure, with severe mechanical defects in the form of large cracks and voids, and chemical The stoichiometry was not balanced. Heavy ions of elements that form alloys such as iron and nickel were detected. Voltage tests showed that these samples behaved like semiconductors, starting to leak current at about 200 to 300 volts. Example 3 Except that the temperature was varied between 5 and 15°C and the current density was varied between 0.02 and 0.05 A/ ^cm2 ,
Anodization was carried out in the same manner as in Example 1. Analysis results show amorphous structure, balanced stoichiometry,
It showed a withstand voltage of over 700V per mil. As expected, due to the high temperatures, the only difference between the samples anodized in Example 1 and Example 3 was the thickness of the anodized coating. This result is consistent with the chemical kinetic conclusion that the formation of the anodic oxide layer is dependent on temperature and activation energy. Example 4 A sample panel of 6061-T6 aluminum alloy was placed in a mixture of sulfuric acid and citric acid at a temperature of 5°C.
Anodization was performed at a current density of 0.023 A/cm ² . The analysis results after anodization showed a mixed structure and cracks similar to the Example 2 sample. This result indicates the need to modify the structure and phase transition of the anodized coating using a leveling agent such as urea or a stabilizer such as Ludox, as described for the preferred embodiment of the bath of the present invention. It is something. Example 5 Several experiments were carried out on samples without citric acid in the anodization bath to demonstrate the effect of ecunic acid as a complexing agent on the elemental composition and withstand voltage of the anodization coating. Mass ion spectroscopy and wave dispersive X-ray analysis showed the presence of heavy ions such as iron, titanium, and nickel.
Withstand voltage tests revealed the current leakage characteristics of ionic conductive solids. Therefore, to obtain an anodized coating that behaves as a true insulator, the chemical mixture of the anodization bath contains a complexing agent that selectively removes heavy ions from the coating during anodization. It is preferable. Example 6 The sample panels of Examples 1, 2 and 4 were placed in an environment with a relative humidity of 85% and a temperature of 85°C. There was no discoloration in the sample prepared in Example 1. The samples of Examples 2 and 4 were discolored and white island-like spots were observed.
Analysis confirmed this to be aluminum hydroxide, the result of a phase transition from aluminum oxide to aluminum hydroxide. As mentioned above, the advantages of the bath according to the invention are believed to be due to the metallurgical and chemical properties of the formulation. This formulation determines the priority in which alternative materials can be used. Complexing agents similar to citric acid can be used. These include short and long chain dicarboxylic acids, EDTA, polyphosphates, and others. Other oxygen-rich leveling agents besides urea can also be used. For example, sugars or sugar derivatives are decomposed at the anode to provide leveling agents and oxygen. In place of lidoxes, inorganic and organic polymeric cations can also be used to stabilize and react non-stoichiometric aluminum oxides.

Claims (1)

[Claims] 1. A method for forming an anodized film on aluminum or aluminum alloy of a substrate for electronic circuit packaging, comprising:
anodizing the aluminum or aluminum alloy while passing a current with an initial current density of 320 A/m ² to about 540 A/m ² , the electrolytic bath containing deionized water, sulfuric acid, citric acid, urea, and ammonium A method for forming an anodic oxide film containing colloidal silica.